In the story of Bleach, you get a character named Ichigo Kurosaki, who is a substitute soul reaper with unbelievable strength...
Diamond Dust Rebellion, however, you see a character of the Soul Society named Toshiro Hitsugaya, one of the youngest squad captains on the force, and view his story in a new light of how he got his position. You also get to see that Toshiro has similar qualities to the main character of Advent Children, Cloud Strife--aloof, harboring guilt for past mistakes...
Diamond Dust Rebellion has quite a bit of action, similar to Advent Children. And what's good, you don't have to know the
WHOLE story of Bleach to watch the movie (A few episodes would be good though). 
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